Photography is the science of capturing an image on a tangible medium by exposure of a light sensitive material to actinic radiation and subsequent processing of the material to produce a visible image. Typically, silver halide is utilized as the light sensitive component of the light sensitive material. Upon exposure, it forms what is known in the art as a latent image, which is the invisible precursor of the useful visible image that appears during photographic processing. The latent image, and more specifically the metallic silver which comprises the latent image, serves to catalyze the reduction of silver ions to silver metal during processing, thus forming the visible image in black and white photographic materials, and forming dye precursors to the visible image in color negative or color reversal photographic materials.
Because the formation of images in photography is dependent upon the exposure of a light sensitive material to actinic radiation, it follows that the formation of images can be impacted by either the level of actinic radiation or the inherent sensitivity of the light sensitive material. The level of actinic radiation--i.e., the brightness of the scene that is to be recorded--is often outside of the control of the photographer, except perhaps to the extent that it may be partially controlled by the use of flashes and the like. The sensitivity of the light sensitive material, on the other hand, may be selected by the photographer to record an image under a given set of conditions.
Current photographic materials exhibit sensitivities that are much higher than their predecessors. However, the industry remains focused on improving the sensitivities of its products even further.
It has been recognized in the art that photographic sensitivity can be increased by adjusting the pH and/or the pAg of a silver halide emulsion. It has also been known that enhanced photographic sensitivity can be obtained by the addition of certain types of compounds called chemical sensitizers to photographic emulsions. Transition metal complexes such as platinum, iridium, osmium and rhodium complexes have been utilized to improve sensitivity. Often, these complexes or similar ones are added during the formation (precipitation) of the silver halide grains which serve as the light sensitive component of the light sensitive material. More often, though, they are added after the formation of the silver halide grains in a step(s) called chemical and spectral sensitization. It is during this step(s) that compounds added to the emulsion can interact with the surfaces of the silver halide grains to affect the grains' photographic properties.
One class of compounds typically added during chemical or spectral sensitization to improve sensitivity is the reduction sensitizers. Reduction sensitization is the sensitization of a photographic emulsion by application of a reduction sensitizer. It is also the process of improving a photographic emulsion's sensitivity by forming the emulsion under certain conditions. Specific examples of reduction sensitizers, such as dimethylamine borane, stannous chloride and hydrazine, are discussed by S. Collier in Photographic Science and Engineering, 23, 113 (1979). This reference also discusses various processes and conditions which, when present during emulsion formation, produce reduction sensitization. Such processes and conditions include the ripening of silver halide grains under conditions of high pH (pH 8-11) or low pAg (pAg 1-7).
Some of the more useful reduction sensitizers known belong to a class of compounds called alkynylamines. The photographic activity of this class has been extensively explored in the art and is described in, for example, U.S. Pat. Nos. 4,378,426 and 4,451,557. Alkynylamines are also described in allowed U.S. Pat. Nos. 5,399,479 and 5,389,510.
Although reduction sensitizers such as the alkynylamines are useful for improving sensitivity, they are also known to have the potential for causing an increase in a photographic emulsion's fog levels. Fog is an indiscriminate and undesirable reduction of silver ions to silver atoms. The silver atoms give rise during development to a generally or locally developed density that is not associated with the action of the image forming exposure; and it is this density which is termed fog.
To correct this deficiency of alkynylamines and other reduction sensitizers, it would be desirable to combine them with compounds capable of countering their adverse characteristics. This would enable the construction of photographic emulsions which exhibit not only improved sensitivity, but an adequate resistance to fog formation. In allowed U.S. patent application Ser. No. 08/168,892 (Docket 66,245PCC-01), discussed above, alkynylamines are utilized in combination with thiosulfonate and sulfinate antifoggants to improve the stability of a photographic emulsion during its storage (incubation). However, this combination has proven to be inadequate for providing sufficient reduction sensitization while controlling both incubational and fresh fog.
In addition to thiosulfonates and sulfinates, other types of antifoggants and stabilizers are known in the art. Examples of these compounds can be found in Research Disclosure 308119, published December 1989. Also, in U.S. Pat. No. 3,300,312, sulfosalicylic acid compounds are described which eliminate spot defects in photographic emulsions caused by metal contamination. Similarly, in U.S. Pat. No. 3,236,652, certain disulfonaphthalene and disulfocatechol salts are described as stabilizing the sensitometric properties of a photographic emulsion against the effects of transition metals. These compounds, however, fail to provide sensitization or antifogging to fresh emulsions.